1. Field of the Invention
The invention relates to infrared, in particular matrix, detectors, of the type produced according to a monolithic structure with pyroelectric materials.
2. Discussion of the Background
In infrared imagery, it is usual to use pyroelectric material to produce the detecting structure itself, and to add this detecting structure to a charge transfer multiplexer or in abbreviated form CCD ("Charge Coupled Device").
The CCD multiplexer is produced on a semiconductive substrate, separate from the detecting structure, with a base of pyroelectric materials, so that the infrared detecting unit constitutes a hybrid structure.
Such infrared detecting units are described in patent document GB 2 200 246 and in the article "Ambient temperature solid state pyroelectric IR imaging arrays" of N. BUTLER, J. McCLELLAND and S. IWASA, published in SPIE vol. 930, Infrared Detectors and Arrays, 1988, pages 151 to 163.
These infrared detecting units are generally surface units making it possible to produce an image comprising a plurality of elementary points placed in lines and in columns. These elementary points, also called pixels, correspond to a given element or given zone of the pyroelectric material.
Generally, a layer of material absorbing the infrared radiation is in contact with the pyroelectric elements. The thermal energy produced by the dissipation of the infrared radiation is sent to the pyroelectric elements, which produce electric charges which are thus proportional to the intensity of the radiation to which the corresponding pixel has been subjected.
The reading of the amounts of charges called "signal charges" produced by each of the pixels, during an exposure phase of the detector, is performed with a read circuit to which all the various amounts of signal charges (or equivalent values) are transferred successively with the CCD multiplexer.
FIG. 1 diagrammatically illustrates the manner by which the transfer of charges to a read circuit in a standard infrared detecting unit is performed. The detecting unit comprises a detecting surface formed by n pyroelectric zones Zl to Zn (n being equal to 16 in the example) placed in lines L1 to L4 and in columns C1 to C4. The detecting surface is associated with a CCD type multiplexer comprising as many shift registers called column registers as there are columns C1 to C4. Each column register R1 to R4 comprises as many stages ET1 to ET4 as there are lines L1 to L4 so that stage of a column register corresponds to each pyroelectric zone Zl to Zn or pixel. Each pyroelectric zone Zl to Zn is connected to the corresponding stage of a column register with an acquisition and charge transfer circuit, called intermediate transfer circuit CTl to CTn. The function of each intermediate transfer stage is to collect the signal charge (or an amount of charges proportional to this signal charge), and to transfer it in stage ET1 to ET4 of corresponding column register R1 to R4.
There therefore exist as many intermediate circuits CTl to Ctn as there are pyroelectric zones.
According to a usual operation, the signal charges produced at the level of each pyroelectric zone Zl to Zn (or a corresponding value) are transferred in the input stage of the intermediate transfer circuit, by which they are then transferred to corresponding reading stage ET1 to ET4 of register R1 to R4. This transfer of charges is performed in a first direction represented by an arrow 4. Since all the signal charges are charged in stage ET1 to ET4 of a column register R1 to R4, these column registers are actuated to transfer these charges in the direction shown by arrow 5, to a reading stage EL1 to EL4 of another shift register called read register RL.
Each column C1 to C4, i.e., each column register, corresponds to a reading stage EL1 to EL4 of read register RL. Thus, for example, the first transfer step of the column registers has the effect, in reading stages EL1 to EL4, of charging the charges originating respectively in pyroelectric zones Z13, Z14, Z15, Z16.
When all these charges are contained in read register RL, the latter, in turn, is actuated to transfer these charges into a read circuit CL, which performs the reading one after the other, and delivers, for each of these charges, an output signal, generally in the form of a voltage proportional to the value of each of the charge amounts. This operation is repeated until the charges produced in the pixels of first line L1 are in turn transferred, in parallel, into read register RL, then transferred by the latter into read circuit CL.
Such infrared detecting units with hybrid structure are relatively difficult to produce, and as a result have a very high cost, in particular because of difficulties linked to the assembly of the detecting structure with the semiconductive structure. Thus, for example, in some cases, the pyroelectric zones consist of as many separate elements as there are elementary image points or pixels. Further, these pyroelectric elements are to be carried by the semiconductive substrate, on which is formed the CCD multiplexer, by contacts that are electrically conductive but thermally insulating. The use of such connecting contacts is necessary to prevent the thermal energy produced in the absorbent surface of the infrared radiation from being sent to the semiconductive substrate, which would have the effect of degrading the operating characteristics of the CCD multiplexer.
Another embodiment is described in the article of M. OKUYAMA et al., "PYROELECTRIC IR-CCD IMAGE SENSOR," published in Ferroelectrics, 1989, vol. 91, pages 127-139. This document describes an infrared detector in which a detecting structure with a base of pyroelectric material is added to the CCD multiplexer, i.e., to the semiconductive substrate, by a dielectric layer which assures the thermal insulation.
FIG. 2 is a perspective view with a partial section. This FIGURE reproduces a FIGURE of the document cited above, showing the detecting structure with a base of pyroelectric material, connected to a semiconductive substrate by a dielectric coupling layer; the latter producing a thermal insulation between the substrate and the detecting structure.
In FIG. 2, semiconductive substrate SS carries a series of electrodes which constitutes an intermediate transfer circuit such as those mentioned with reference to FIG. 1. In a similar manner to that already explained with reference to FIG. 1, from an injection electrode referenced 50, the charges are transferred in the direction of transfer shown by arrow 4, in the direction of a column register stage (with four phases) represented by four electrodes referenced 51. As in the example of FIG. 1, direction of transfer 4 is parallel to the pixel lines: i.e., the series of electrodes shown in FIG. 2 relates to a single pixel, but in fact, such series of electrodes repeat to the left and to the right, and also in depth in the direction of the columns, to form a complete detector. The amount of charges transferred to stage 51 of the column register is controlled at the level referenced 52, which constitutes the input stage. These charges are then to be transferred in the direction shown by arrow 5, to be charged in a read register stage (not visible in FIG. 2) similar to that described with reference to FIG. 1.
Semiconductive substrate SS, and the series of electrodes that is repeated in the direction of the lines, and also at a right angle to the latter in the direction of the columns, constitute the CCD multiplexer part, on which detecting structure SD is peaced.
The detecting structure comprises a layer of pyroelectric material MP, itself covered by a layer AI intended to absorb the infrared radiation. In this embodiment, the pyroelectric zones or pixels are embodied by metal electrodes (not shown in FIG. 2) formed on the layer of pyroelectric material, on the side of the semiconductive substrate. The detecting structure is not in contact with the electrodes of the CCD multiplexer, from which it is insulated electrically and thermally by a layer of a dielectric coupling material MC and by an insulating layer of SiO.sub.2 ; coupling layer MC being in contact with the pyroelectric material and the insulating layer being in contact with electrodes It should be noted that the coupling between the detecting structure and the CCD multiplexer is made more significant at the level of input stage 52, by the fact that the insulating SiO.sub.2 layer is interrupted at the level of this input stage to leave room for dielectric coupling layer MC.
Such an embodiment exhibits an advancement, in particular in that it does not require using contacts that are electrically conductive and thermally insulating between the semiconductive substrate and each pyroelectric zone corresponding to a pixel. However, although advantageous, this embodiment still exhibits drawbacks attached to hybrid type structures, because, actually, the CCD multiplexer, on the one hand, and the detecting structure, on the other hand, are produced separately.
A much more striking improvement of the infrared detectors with a base of pyroelectric material results from using pyroelectric materials having low thermal conductivities k, for example, less than 1 W/m.K. Such a use of pyroelectric materials with low thermal conductivities is described in a French patent application no. 89 08799, now U.S. Pat. No. 5,087,816. This document indicates that it is possible to use polymers (for which generally k is less than 0.2 W/m.K) such as:
--polyvinylidene fluoride (PVDF),
--polyvinylidene fluoride - trifluoroethylene (PVDF-TrFE),
--polyvinylidene fluoride - vinyl acetate (PVDF-VAc),
--polyvinylidene cyanide - vinylidene fluoride (PVDCN - VDF).
It is generally possible to use composite materials (for which k is less than 1 W/m.K) such as mineral charge compounds of great thermal conductivity mixed in a polymer matrix of low thermal conductivity. For example, 60% by weight of PZT in polyimide gives a coefficient k of about 0.9 W/m.K.
The advantage of such a pyroelectric material with low thermal conductivity resides in that it can constitute the detecting surface by an approximately uniform layer able to be produced or deposited on the semiconductive substrate already carrying the CCD multiplexer. In fact, the pyroelectric layer is produced on electrodes whose surface and position approximately define the surface and the position of a pyroelectric zone corresponding to a pixel.
Each of these electrodes, called "lower electrode" in the description below, thus constitutes one of the plates of a pyroelectric capacitor to which each pixel or pyroelectric zone can be compared; the other plate of the pyroelectric capacitor can be constituted on the other face of the pyroelectric layer, for example, by an electrically conductive layer forming an electrode called upper electrode and which is common to all the pyroelectric zones.
Infrared detecting units with a base of pyroelectric material are thus produced having a monolithic structure, and whose production is then considerably simplified.
Relative to the hybrid structure shown in FIG. 2, such a monolithic infrared detector can comprise the same semiconductive substrate and a similar series of electrodes to constitute a CCD multiplexer similar to the one in FIG. 2. On the other hand, the layer of pyroelectric material with low thermal conductivity, although produced or deposited on the CCD multiplexer, can appear in a manner similar to that of the pyroelectric layer of FIG. 2; except for the fact that while, as in the hybrid type embodiment, the layer of pyroelectric material rests on the semiconductive substrate by a dielectric coupling layer MC, while in the case of the monolithic structure, coupling layer MC is replaced by an electrically conductive layer intended to form the lower electrode mentioned above.
However, in the case of the monolithic infrared detector having a layer of pyroelectric material with low thermal conductivity, since this layer is produced directly on the semiconductive substrate, it is necessary to subject it to a so-called "polarization" operation, intended to produce the crystalline orientation. This polarization consists in applying, between the two electrodes of each pyroelectric capacitor or pyroelectric zone, an electric voltage having a suitable polarity and an amplitude which can reach, for example, 1000 volts per 10 micrometers of thickness of the pyroelectric material.
The polarization of the pyroelectric material, within the framework of a monolithic infrared detector, exhibits the drawback of not being very simple to produce, because it requires gaining access to all the lower electrodes of all the pyroelectric capacitors. For this purpose, it is possible, for example, with standard microlithography techniques, to connect all the lower electrodes to one another and to an outer electrode, then to apply the polarization voltage between this outer electrode and the upper electrode mentioned above; then, next by the microlithography techniques, to eliminate the connections between all these lower electrodes to make possible the normal operation of all the pyroelectric capacitors.
This solution is certainly achievable, but it is long, delicate, and consequently costly.
Another problem attached to the infrared monolithic type detector lies in the fact that, relative to other pyroelectric materials, for the same thermal energy, the amount of electric charges produced by the pyroelectric materials with low thermal conductivity is clearly smaller, and as a result does not make it possible to obtain as good a thermal resolution.